1. Field of the Invention
This invention relates to improved current controlled transfer switches for magnetic bubble domains, and more particularly to such transfer switches in which the magnitude of the transfer current is substantially reduced over that of conventional transfer switches.
2. Description of the Prior Art
In magnetic bubble domain storage systems, transfer switches are often required. These switches are used to change the normal propagation path of a bubble domain so that it will move into a new propagation path under the control of an input supplied to the switch. This input is a magnetic field which can be supplied, by, for instance, another bubble domain, a current pulse, or the stray magnetic field of a magnetic element. Common transfer switches utilize switching currents to achieve the transfer operation and either apply a current pulse to retain the bubble domain in its normal propagation path (such switches may be termed "drive-to-retain" switches) or apply a current pulse to effect the transfer operation in order to send a bubble domain to a new propagation path (these switches may be called "drive-to-transfer" switches).
The major difficulty in achieving operation of very high density magnetic bubble devices using current controlled transfer switches lies in providing adequate switching current for moving very small bubble domains. Generally, as the size of the bubble domain decreases, the magnetization 4.pi.M of the bubble medium increases and the amount of the magnetic drive field H.sub.xy required to propagate bubbles also increases. Scaling laws indicate that as the bubble diameter decreases, the required switch current for a given transfer switch design remains substantially constant. However, as the bubble domain diameter becomes smaller, the minimum linewidths on the structural features of the switch also decrease. Since the required switch current remains substantially constant, the current density required for switching becomes very high as the bubble diameter and hence the structural features on the magnetic chip are scaled downward. This in turn becomes a problem due to electromigration effects where high current densities adversely affect the properties of the current carrying conductors.
In order to avoid this problem, switches operating on principles other than current control have been devised. For example, switching action can be achieved by a proper sequencing of the rotating magnetic drive field used for propagating bubble domains. This is shown in, for instance, W. F. Beausoleil et al, IBM Technical Disclosure Bulletin, Volume 15, No. 7, December 1972, p.2093, as well as in U.S. Pat. Nos. 3,613,058 and 3,543,252. While this appears to be an ideal solution since no transfer current is required, considerable effort is required to develop appropriate geometries for the transfer switch elements and for the bubble propagation elements in the chip in order to provide acceptable operating margins. Furthermore, the fact that all switches in a given bubble domain module will necessarily see the same drive field sequence places substantial constraints on the design of the chips as well as on the entire system.
Another approach to providing bubble path selection by varying the magnetic drive field is illustrated in U.S. Pat. Nos. 3,530,446 and 3,541,535. In these patents, the amplitude of the rotating drive field is changed in order to change the direction of bubble propagation or to select the channel in which propagation occurs. These approaches are not as desirable, however, since they often require either a change in the geometry of the elements in order to provide the desired propagation directions, or variations in the material properties of the propagation elements used to move the bubble domains. This in turn requires additional fabrication steps and the use of at least two materials having different magnetic properties.
Current controlled transfer switches appear to be the type of transfer switches which have been most extensively developed by the industry. These include both the "replicate" type of switch in which a bubble domain is split into two parts to enable retention of the stored information, and the conventional type of transfer switch in which the stored bubble domain is not split. Many of these switches have been designed to be fabricated entirely of a single material in order to enable a single masking step fabrication process. This single level metallurgy (SLM) is a desirable feature, especially when very small magnetic bubble domains (of 1 micron diameter and less) are to be used. In particular, U.S. Pat. No. 3,984,823 describes a replicate-type transfer switch in which a sequence of current pulses is applied to stretch a domain and then cut it to effect replicate/transfer operation. This switch includes means for maintaining the magnetic drive field orientation in a fixed direction during the stretching and replicate operation. This is considered to provide advantages since it permits a stop-start operation in which detection can occur in the absence of a magnetic drive field reorientation, which in turn benefits signal-to-noise ratio and logic-circuit design. If the drive field is terminated altogether during the stretch-replicate operation, a separate magnetic field of fixed orientation is applied.
While U.S. Pat. No. 3,984,823 describes a transfer switch in which a control current is used in combination with a modification of the magnetic drive field during the transfer operation, it is not directed to a switch which requires only small amounts of transfer current. In the switch of that patent, the magnitude of the drive field remains constant and its orientation is fixed during the transfer operation. While this provides greater tolerances in phasing, it does not reduce the required currents for a switch using very small magnetic bubble domains.
Accordingly, it is a primary object of the present invention to provide an improved magnetic bubble domain transfer switch which requires small amounts of transfer current.
It is another object of this invention to provide a magnetic bubble domain transfer switch which can be scaled down to transfer micron and submicron bubble domains without requiring adversely high currents.
It is another object of the present invention to provide a technique for operating magnetic bubble domain transfer switches of any design in a mode which requires minimal transfer current amplitudes.
It is a further object of the present invention to provide magnetic bubble domain transfer switches which can be fabricated by single level metallurgy techniques and which require minimal transfer current amplitudes.
It is a further object of the present invention to provide magnetic bubble domain chips using minimal current amplitudes for transfer switches with drive field sequences which do not adversely affect margins of other devices in the bubble chip.
It is a still further object of the present invention to provide improved current-controlled SLM bubble domain switches where problems due to magnetization reversal are minimized.
It is a further object of the present invention to provide an improved magnetic bubble domain chip using a non-conventional applied drive field cycle which is continuous.
It is another object of the present invention to provide a magnetic bubble domain chip using a magnetic drive field cycle which aids in the design of bubble sensors so that noise signals can be minimized.
It is a further object of the present invention to provide improved operation of magnetic bubble domain devices by techniques which can be applied to many different switch and propagation element designs.